<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>下拉菜单</title>
    <link rel="stylesheet" href="bootstrap.min.css"/>
</head>
<body>
    <div class="container">
        <div class="dropdown" id="mydropdown">
            <button class="btn btn-default" id="dLabel" type="button" data-toggle="dropdown">
                下拉菜单
                <span class="caret"></span>
            </button>
            <ul class="dropdown-menu" role="menu">
                <li>1</li>
                <li>1</li>
                <li>1</li>
                <li>1</li>
                <li>1</li>
            </ul>
        </div>
    </div>

    <script src="jquery-1.11.1.js"></script>
    <script src="bootstrap.min.js"></script>
    <script>
        $("#mydropdown").on("show.bs.dropdown",function(e){
            alert("hello");
        })
    </script>
</body>
</html>